Ix] EXCITED RADIO-ACTIVITY 271 
total number of ions produced during the first change is not much 
more than one per cent. of that produced in the second change. 
If, for example, the initial activity be taken as due to the radia- 
tion from the first change, the activity due to the first change 
alone should fall off in an exponential law with the time, 
following the dotted curve D shown in Fig. 50. The area 
EABE serves as a comparative measure of the total number 
of ions produced by the first change, and this area is seen to 
be small compared to the corresponding area included by the 
main curve C. 
There is, however, no reason to suppose that the first change is 
accompanied by any ionizing radiation at all. The initial activity 
observed is due to the fact that some of the deposited matter has 
undergone change before the rod is tested; for it will be shown 
that the experimental curve obtained can be completely deduced 
if the first change is supposed to take place without any emission 
of tonizing rays, but that ionizing rays are emitted in the second 
change. 
It has been shown that after removal of the body for a time 7 
the number of particles g which have undergone the first change 
but not the second change is given by 
Ay No 
pean eee ee 
where 2, 1s the constant of decay in the first change and A, for the 
second change. 
Since it is supposed that only the second change gives rise to 
a radiation, the activity at any time 7’ after removal is propor- 
tional to g. The value of q passes through a maximum when 
Noe @F— Nie-*2=0, 
: rz 
ze. when — = e-Aia~ry) TD 
1 
Now it is known, from experiments for a long interval of ex- 
posure, that in the second change the activity falls to half value 
in 11 hours, 2.e. },='063, when the time is expressed in hours. 
Since the maximum activity 1s reached when 7’=220 minutes 
approximately, the value of \,=°75. Substituting the values of 
